First of all, a description will be given, using FIGS. 1A-1D, of a concept of the SCAT which the present inventor proposes.
A cordless power tool, such as a power screwdriver, a power drill or an impact tool, including a motor which generates a rotational power, is configured in such a way as to, after causing a deceleration mechanism to reduce a rotating speed of the motor, transmit the rotational power to a tipped tool.
In FIG. 1A, 20 depicts a cordless power tool, which is configured of a main body 20A and a handle 20B. A tool 30 is attached to a leading end of the main body 20A. An end of the handle 20B is connected to the main body 20A, and a battery device 10 is attached to the other end.
For all such cordless power tools, a rated voltage (volt, hereafter abbreviated as V) and a current capacity (ampere hour, hereafter abbreviated as Ah) are set by their maker. The rated voltage (V) is determined based on a size of a rotational power transmitted to the tool and a voltage necessary to drive a motor which generates the rotational power. Also, the current capacity (Ah) is determined based on a size of a load current of the motor and a specification of a time for which the tool can be used continuously. For example, a power tool having a battery device of 3 Ah attached thereto has a property which can supply a current of 3 A to the motor continuously for one hour.
Such a rated voltage and a current capacity are determined for every tool by the maker, and a user is not at liberty to change or alter their values.
In response to this, the SCAT proposes a new concept power tool configured in such a way that, although the rated voltage (V) is determined by the maker, the user can optionally select the current capacity (Ah).
This kind of new concept is desirable in that a variety of user needs can be met with one cordless power tool. For example, when using a power tool in a narrow space in a ceiling or the like, the user will desire that the power tool is as light in weight as possible rather than being large in size. However, nearly half a weight of the hitherto known cordless power tool is occupied by a cell pack, that is, the battery device and, as the power tool is manufactured in such a way that only a cell pack appropriate to the rated voltage and current capacity of the power tool is attachable to it, it is impossible to change the weight of the power tool in any operation.
Meanwhile, when carrying out the same operation continuously for a long time, the user will desire a power tool which can be used without charging the cell pack frequently. However, in the hitherto known power tool, as the current capacity is set in advance, it is impossible to use a cell pack of which a value of the current capacity differs according to the operation.
As there are many models of cordless power tool, of course, it is possible to use a tool whose specifications differ according to the operation, but the user will not desire to prepare a large number of power tools or carry them along to an operation site.
The SCAT meets such a variety of user needs. A description will be given of a difference between the hitherto known battery device and the battery device using the SCAT, by suggesting an example in which the rated voltage of the power tool is 18V and the current capacity is 3 Ah.
FIG. 1B shows a configuration of a hitherto known battery device in a case of using a nickel-cadmium cell having a nominal voltage of 1.2V as a battery cell. The battery device is configured by connecting 15 cells C1 to C15 in series and housing them in a cell pack container 10A.
Meanwhile, in a case of using a lithium cell as the battery cell, as the lithium cell has a low nominal voltage of 3.6V and a high current capacity of the order of 1.5 Ah, as shown in FIG. 1C, by connecting five cells C11 to C15 connected in series and five cells C21 to C25 similarly connected in series, a total of 10 battery cells are housed in a cell pack container 20A, configuring the battery device.
In response to this, in a case of using the SCAT, as shown in FIG. 1D, the maker prepares a cell assembly housing a number of cells necessary to generate a rated voltage of the cordless power tool. For example, in a case of configuring a cell assembly with lithium cells, a cell assembly 100A is configured by serially connecting five battery cells C11 to C15 having a nominal voltage of 3.6V and housing them in a container. In the same way, a cell assembly 100B is configured by connecting battery cells C21 to C25 in series and housing them in an assembly container. The cell assemblies 100A, 100B, . . . 100N are configured in such a way as to be connected in parallel when stacked one on another.
The user, when using one cell assembly, can use it as a 1.5 Ah battery device and, when using two cell assemblies, can use them as a 3 Ah battery device. That is, it is possible for the user to selectively determine a value of the current capacity (Ah) of the cordless power tool.
In order to manufacture a cell assembly using the SCAT, it is desirable to use a lithium cell having a high normal voltage and a low current capacity as a cell. The reason for this is that it reduces a weight of the cell assembly and enables a user's more precise selection of the current capacity.
As used herein, the lithium cell refers to a lithium vanadium pentoxide cell, a lithium manganese dioxide cell and the like, all of which, using a lithium aluminum alloy at a negative electrode, use an organic electrolyte solution. Also, a lithium ion cell generally, using a lithium cobalt oxide at a positive electrode, uses an organic electrolyte solution as an electrolyte solution. In the present specification, for the sake of convenience, an organic electrolyte solution secondary cell including the lithium cell and the lithium ion cell is collectively referred to simply as the lithium cell.
As a hitherto known technology similar to the SCAT, a battery device has already been proposed or developed which is configured in such a way that a plurality of chargeable cells can be connected in parallel, for example, in a portable electronic instrument such as a camera or a personal computer. For example, in Patent Document 1, a cell pack is disclosed which, being used in a camera and the like, enables an attachment thereto of a desired number of auxiliary cells in addition to a main cell. However, in a case of the cordless power tool, as there are technical problems of a different nature from those of office automation equipment and the portable instrument, in order to develop a power tool battery device using the SCAT, it is necessary to solve the technical problems.
First, a description will be given, with reference to FIGS. 2A and 2B, of an example of the hitherto known cordless power tool.
FIG. 2A shows an external appearance of the hitherto known cordless power tool, and FIG. 2B shows an outline of an electrical circuit of the power tool. A power tool 20 such as a power screwdriver, a power drill or a power wrench, including a main body 20A and a handle 20B connected to the main body 20A, has a battery device 10 attached to an end of the handle 20B.
A DC motor 201, which generates a rotational power, and a deceleration mechanism 202, which reduces a rotating speed of the DC motor 201, are housed in a housing of the main body 20A, and a tipped tool 30 such as a drill or a screwdriver is attached to a leading end of the deceleration mechanism 202. In a case of an impact tool, an impact mechanism (not shown), such as a hammer, is provided between the deceleration mechanism 202 and the tipped tool 30. Also, a trigger 203 is provided in the vicinity of a connection between the main body 20A and the handle 20B.
As shown in FIG. 2B, a trigger switch 203A, the motor 201 and a switching element 205 such as an FET are connected in series between both terminals of the battery device 10. A pulse signal having a pulse width modulated by a control circuit 204 is applied to a gate of the switching element 205. Also, a variable resistor 203B of which a resistance value varies in response to an operation of the trigger switch 203A is connected to the control circuit 204, and a configuration is such that a pulse width of an output pulse of the control circuit 204 varies as the resistance value is changed.
At this point, by pulling the trigger 203 in FIG. 2A, the switch 203A in FIG. 2B is closed, and a drive voltage is applied to the motor 201 from the battery device 10 for only a period in which the switching element 205 is on, rotating the motor 201. The rotational power is transmitted to the tipped tool 30 via the deceleration mechanism 202.
By further pulling the trigger 203, the resistance value of the variable resistor 203B varies. By this means, the pulse width of the pulse applied to the gate of the switching element 205 from the control circuit 204 increases. For this reason, a time for which the switching element 205 is on increases, and an average value of the drive voltage applied to the motor 201 increases. Consequently, the rotating speed of the motor 201 can be controlled in accordance with a pulling amount of the trigger 203, and it is possible to control a size of the rotational power transmitted to the tipped tool 30. By switching a switch 206 connected to both ends of the motor 201, it is possible to switch a forward and a backward rotation direction of the motor.
[Patent Document 1] JP-A-2001-229891